This research project is moving into the third year of a four year study, in which the relationship between the size of fat cells, as well as the distribution of cell sizes in an individual sample, is related to a range of other factors. The relationship between cell size/distribution and these factors is hypothesized to be significant in the subsequent development of obesity. Fat cells are obtained from individuals undergoing hernia repair, between early infancy and 4 years of age; the sample size is 225, with 150 of the subjects, under one year at initial examination, followed longitudinally through 30 months of age. The other factors analyzed encompass the following areas: 1) growth status at the time of obtaining fat sample; 2) subsequent growth (for those followed longitudinally); 3) socioeconomic and demographic characteristics of the family; 4) dietary intake at 6 and at 18 months; 5) parental body size and growth status of siblings; 6) in a selected sub-sample, energy expenditure as determined from O2 consumption over a baseline period and heart rate monitoring over a 24-hour period.